Method of producing highly absorbent cellulose

ABSTRACT

HIGHLY ABSORBENT CELLULOSE IS PRODUCED BY ADDING TO THE PULP AN ORGANIC ANIONC OR NONIONIC AGENT AND A CATIONIC RETENTION AGENT WHILE REGULATING THE AMOUNT OF THE CATIONIC SUBSTANCE IN SUCH A WAY THAT THE Z-POTENTIAL OF THE PULP APPROACHES ZERO.

United States Patent @1 ice- 3,677,886 METHOD OF PRODUCING HIGHLYABSORBENT CELLULOSE Lars-Henrik Forssblad, Iggesund, and Per Jerkeman,

Gavle, Sweden, assignors to Korsnas-Marma Aktiebolag, Gayle, Sweden NoDrawing. Filed Dec. 9, 1969, Ser. No. 883,609 Claims priority,application Sweden, Dec. 27, 1968, 17,816/68 Int. Cl. D2111 3/02 US. Cl.162-72 4 Claims ABSTRACT OF THE DISCLOSURE Highly absorbent cellulose isproduced by adding to the pulp an organic anionic or nonionic agent andacationic retention agent while regulating the amount of the cationicsubstance in such a way that the Z-potential of the pulp approacheszero.

BACKGROUND OF THE INVENTION It is known to produce highly absorbentcellulose, socalled fluff, by dry defibration of dried pulp in the formof sheets or endless webs. T o be suitable for fluif, the pulp should beof such a texture that the fibres can be sepa rated from each otherwithout being shortened to any noticeable extent, and it should bepossible to carry out the separation of fibres at a low consumption ofenergy. Further, the fluff obtained should exhibit properties being ofdecisive importance to its use in hygienic products, for which it isintended, such as high bulk, good elasticity, great absorption capacityand absorption rate.

The requirement that the pulp should be easy to defibrate presupposesand intentional reduction of the natural tendency of the fibres toadhere to each other on drying. To achieve said reduction of the bindingcapacity it has previously been proposed to add a cationic organic agentto the pulp in the cellulose mill immediately before or on the wetmachine.

By the addition of neutral compounds, such as nonionic or anionicsubstances, it is possible to obtain the desired reduction of thebinding force of the fibres in an effective and reliable mannerindependently of the substantivity of the substance in question. Theadhesion of high-molecular, organic substances to cellulose fibres isdetermined by colloid chemical factors which will be mentioned briefly.In aqueous suspension, a natural, pure cellulose fibre assumes anegative electric charge which is localized to the fibre surface in theboundary layer between the fibre and the water. In its immediateproximity in the aqueous phase this charge fixes an immovable andsharply defined layer having a positive charge, and said positive layerinduces, in turn, in the water outside it a diffuse, negatively chargedlayer which is movable and moves with the water, if the latter is causedto flow. The difference in potential between said diffuse charge layerand the main mass of the suspension water has been designated asZ-potential and can be determined by means of electrokinetical measuringmethods. The Z-potential which has the same character as the charge ofthe fibre surface, grows and diminishes numerically with the charge,when the latter is changed by charged particles (ions) supplied3,677,886 Patented July 18, 1972 from outside. The Z-potential and thefibre charge reach the value zero simultaneously, and thus theZ-potential gives a valuable indication of the state of charge of thefibre.

SUMMARY OF THE INVENTION Now it has turned out that the negative,electrostatic surface charge of the fibre can be reduced, whollyeliminated or changed to a positive charge, so that the defibration ofthe dry pulp is facilitated and a highly absorbent cellulose or fluff isobtained. The new method consists in that two different-kinds ofsubstances are added to a pulp immediately before or on the wet machine,Whereupon the pulp is dried and the dried pulp is defibrated in a mannerknown per se. One of these substances is a nonionic or anionic, finelydispersed, high-molecular, organic substance having a molecularstructure containing hydrophobic parts, and the other substance is acationic retention agent which is added in such an amount that theZ-potential of the pulp approaches zero.

If the retention agent is added in a suitable amount, it changes thestate of charge of the fibres, so that the fibres are capable ofretaining nonionic and anionic substances on their surfaces. To retainthe nonionic or anionic sub stance on the walls of the fibres to such adegree that the desired effect is obtained it is necessary to proportionthe amount of retention agent such that the charge of the fibres andthus the Z-potential assumes a very low value, i.e. in the proximity ofnought. By adjusting the Z-potential of the fibres it is thus possibleto achieve that sufficiently large molecules of a substantiallyhydrophobic character are caused to adhere to the walls of the fibres sothat the force of the binding fibre-to-fibre is reduced at the drying ofthe pulp. The Z-potential may be determined by an electrokineticalmeasurement as known per se. 7

A number of substances are known as retention agents in otherconnections. Examples of such agents are several metal salts withpolyvalent cations, such as Al, Ce La, Th Zr certain finely di-videdminerals such as chrysotil asbestos and certain nitrogenous organicsubstances such as alkyl amines, al'kyl diamines, polyimines,alkoxylated derivatives thereof, and others.

DETAILED DESCRIPTION OF THE INVENTIO The method according to theinvention will be illustrated by the following examples.

Example 1 A nonionic (neutral) parafiine emulsion (Mobilcer HM)containing 0.4 g. of paratiine was added, while agitating, to 5 litersaqueous suspension of a bleached pine sulphate pulp (2% concentration).After 10 minutes and while continuing the agitation, an aluminumsulphate solution (10 g. of Al (SO per liter) was added in portions,until a sample withdrawn showed that the Z-potential had assumed thevalue zero. For that 50 ml. of aluminum sulphate solution was needed,i.e. 0.5% Al-sulphate, calculated on the weight of the dry pulp. TheZ-potential was determined by means of a Streaming Current Detector madeby Waters Associate Inc., Framingham, Mass, U.S.A.

The pulp suspension was drained on a suction filter to form a number offibre sheets containing about 700 g. of dry pulp per sq. In. The sheetswere drained by pressing to a dry content of 35% and were then dried to93% dry content. From another batch of the same bleached su1- phate pulpwithout added chemicals fibre sheets were formed in the same way anddried to 93% dry content. Both kinds of fibre sheets were dry defibratedin a laboratory shredder equipped with means for measuring theconsumption of energy during the dry defibration. In the following tablethe consumption of energy and the bulk of the flutf obtained arecompared.

Example 2 While agitating, 0.06 g. of the retention agent DiaminacetatBG (Liljeholmens Stearinfabriks AB) was added to liters aqueoussuspension of bleached sulphite pulp (2% concentration), and afterminutes 0.45 g. of Tergitol Anionic 4 (sodium tetradecyl sulfate,produced by Union Carbide Corp.) was added, while continuing theagitation. Previously it had been ascertained by a preliminary test thatthe Z-potential zero was reached when adding 0.06% of the retentionagent, calculated on the weight of the dry pulp.

In the same way as in Example 1 laboratory-made sheets were produced andflufi" was made therefrom in a laboratory shredder. In the followingtable the consumption of energy at the dry defibration and theproperties of the product obtained are compared with correspondingvalues for the same pulp without added chemicals. The so-called knotnumber was determined by causing an aqueous suspension of a certainamount of fluff to pass through an electronic counter, in which thenumber of particles above a certain size were counted by a photocellconnected to a computer.

Example 3 In the pulp vat of the pulp mill immediately before the wetmachine, there was added to a bleached sulphate pulp of 2% concentration0.4%, calculated on the weight of dry pulp, of nonyl-phenoxy-polyglycolether (Berol EMU-04) emulsified in water, and 0.05% (calculated on theweight of dry pulp) of a retention agent consisting of an ethoxylatedalkyl amine (Berol EMU-302). On the basis of Z-potential determinationsin reference samples the amount of amine had been determined such thatthe Z-potential of the pulp was zero.

In the following table the consumption of energy at the dry defibrationand the properties of the product obtained are compared withcorresponding values for the same pulp without additions of chemicals.

TABLE Without With additions additions Consumption of energy at drydefibration of the pulp sheet in laboratory equipment of thespiked-roller type, kwhJton. 230 180 Number of undisintegrated knots pergram of flufi (knot number) 230 Long fibre traction (retained on ascreen of 18 mesh in a fibre grouping apparatus according to Bauer-McNett) percent 25 39 Absorption capacity, g. water/g.fiufi 12. 012. 2

What we claim is:

1. A method of producing highly absorbent cellulose in the form of afluff, comprising adding to a pulp, immediately before or on the wetmachine, a finely dispersed high molecular Weight substance containinghydrophobic parts and selected irom the group consisting of an organicanionic agent and an organic nonionic agent, in an amount of ODS-5%,calculated on the dry weight of the pulp, and a cationic retention agentconsisting of a water soluble salt of a polyvalent metal selected fromthe group consisting of A1 Ce, Th, Zr and Let in an amount of 0.01-5% ofthe anhydrous salt thereof, calculated on the dry weight of the pulp,while regulating the amount of the added cationic substance in such away that the Z-potential of the pulp approaches zero, drying the pulp,and then defibrating the dry pulp.

2. A method according to claim 1, wherein the high molecular weightfinely dispersed substance is a nonionic paraifine.

3. A method of producing highly absorbent cellulose in the form of afluff, comprising adding to a pulp, immediately before or on the wetmachine, finely dispersed sodium tetradecyl sulfate in an amount ofODS-5%, calculated on the dry weight of the pulp, and a cationicretention agent consisting of diamine acetate, in an amount of0.014110%, calculated on the dry weight of the pulp, while regulatingthe amount of the added cationic substance in such a way that theZ-potential of the pulp approaches zero, drying the pulp, and thendefibrating the dry pulp.

4. A method of producing highly absorbent cellulose in the form of afluif, comprising adding to a pulp, immediately before or on the wetmachine, a finely dispersed nonylphenoxy polyglycol ether in an amountof 0.0S5%, calculated on the dry weight of the pulpand a cationicretention agent consisting of an ethoxylated alkyl amine in an amount of0.010.10%, calculated on the dry weight of the pulp, while regulatingthe amount of the added cationic substance in such a way, that theZ-potential of the pulp approaches zero, drying the pulp, and thendefibrating the dry pulp.

References Cited UNITED STATES PATENTS 3,016,325 1/1962 Pattillochl62-164 2,432,128 12/1947 Schlosser et a1. 162-l58 X 2,789,902 4/1957Seymour et al 162-72 2,957,797 10/1960 Nakayama 162158 3,501,373 3/1970Illingworth 162-72 3,536,697 10/ 1970 Kolosh et al. 16272 S. LEONBASHORE, Primary Examiner F. FREI, Assistant Examiner U.S. Cl. X.R.

